This invention relates generally to vaporization heat transfer devices and, more specifically, to the structure and method of constructing multiple groove wicks which comprise the capillary assisted evaporator surface in these devices. The enhanced surface of this invention can be used in any device which uses capillary forces to spread liquid over an evaporating surface, e.g., 2-phase mounting plates, heat pipes, solar collectors and, generally, heat sink devices.
The use of microgrooves as the wick, or capillary structure, for capillary assisted evaporator surfaces is well known in the art. Performance of these devices can be considered in two parts. The first part is the distribution of the liquid onto the grooved, evaporative surface. The second is the mechanism of transferring heat to the liquid film, which causes the liquid to evaporate. The liquid distribution is governed by the capillary pumping ability of the grooves in the evaporative surface and by the means for getting the liquid to the grooves which may be another interfacing wick. The mechanics of calculating the performance of the capillary pumping ability of the grooves or wick is well covered in the literature, see, e.g., Frank, S., "Optimization of Grooved Heat Pipe," Intersociety Energy Conversion Conference, 1967 or U.S. Pat. No. 3,598,180 to R. E. Moore, Jr. Evaporation is represented by a heat transfer coefficient, whose magnitude is determined by the groove geometry and the liquid film distribution. Groove geometry is also discussed in the literature. See e.g., Harwell, W., Kaufman, W. B., Tower, L., "Re-Entrant Groove Heat Pipe," American Institute of Aeronautics and Astronautics 12th Thermo Physics Conference, June 1977 and U.S. Pat. No. 4,274,479 to Eastman. The latter reference teaches several different cross sectional configurations for grooves and further teaches narrowing or blocking of the grooves to assure that the capillary pumping pressure is maximum at the vapor liquid interface which should occur at the narrowest portion of the groove. However, the problem with the prior art is not a failure to properly describe the generally preferred groove geometry, but it was extremely difficult and expensive to achieve. To achieve the proper groove geometry, the grooves were either extruded or, as taught in Eastman, the grooves were formed on the inside of a heat pipe from metal powder sintered in place around shaped mandrels to form the wick. In both of these methods, the lands or material between the grooves was far too thick for efficient operation. For rectangular grooves most of the heat transfer occurs in a small area where the liquid meniscus attaches to the groove top. There is very little heat transfer from the land area between the grooves. With groove spacing equal to the groove width, mathematical analysis shows that the heat transfer coefficient increases as the groove width and spacing decrease. A rigorous analysis of capillary performance and evaporative heat transfer suggests that the evaporative surface is improved by: (1) decreasing groove width to get higher capillary pressures; (2) increasing the number of grooves per centimeter to allow higher heat transfer coefficients (same heat per groove, with more grooves); (3) increasing the groove cross-sectional area and hydraulic diameter to provide lower flow pressure loss; and (4) making more effective use of the land area for heat transfer.
It is an object of this invention to provide an enhanced capillary assisted evaporator surface by providing a small capillary radius at the groove opening to optimize the capillary pumping, while maintaining a large flow area and, at the same time, significantly increase the heat transfer coefficient by making more effective use of the land area between grooves.